1. Field of the Invention
The invention generally relates to the recovery of nutrients during the treatment of animal waste. In particular, the invention provides an economical, integrated system for both nutrient recovery and biogas scrubbing from the effluent and gas that results from digestion of animal waste.
2. Background of the Invention
The management of animal waste is a problem of ever increasing magnitude. It has been estimated that approximately 250 million tons of dry animal manure are produced yearly in the United States, with large amounts being generated by concentrated animal feeding operations (CAFOs). Historically, animal waste was successfully used as fertilizer on agricultural crop land. However, CAFOs generate more waste than can be disposed of in this manner without causing detrimental nutrient buildup. For example, phosphorus in runoff from such agricultural operations has been identified as a major contributor to water pollution. The use of “lagoons” to contain animal waste is now a common practice. However, there are problems associated with this method as well. Lagoons are a major source of methane gas and odors, and their capacity is also finite. Exporting of animal waste away from the agricultural operation merely shifts the location of the problem without providing a solution and transportation of the liquid waste is not economical.
A recent trend in animal manure management is the renewed interest in using anaerobic digestion (AD) technology for energy production, odor control, and waste water mitigation. As of 2007, more than 100 CAFO digesters had been built in the United States, with the vast majority having been built in just the last 5 years (EPA AgStar, 2007). Many producers have shown a commitment to AD technology because of its strengths in odor abatement, energy production and development of useful co-products such as improved fiber bedding. Although AD is advantageous in regard to methane entrapment, reduction of volatile organics, solids reduction, chemical oxygen demand, vector reduction and pathogen removal, it does not reduce or recover nutrients. This is particularly true for phosphorous, which is found both in the settled solids and the mixed-liquor effluent, and nitrogen, which is found as gaseous or dissolved ammonia as well as in organic form within the mixed liquor and settled solids. In fact, AD can be seen as making ammonia gas emission even more problematic as it converts as much as 25% of the organic nitrogen to an inorganic or ammonia form.
Currently AD designs do not include processes for recovering nitrogen and phosphorous and thus they contribute little to the solution of key air and water quality problems. As a result of this limitation and the capital costs involved, many farmers are not willing to adopt AD technology which is likely why CAFO AD adoption, although growing, is not growing at a rate that would be expected or desired. This is in spite of the fact that CAFOs are under increasing scrutiny in regard to air emissions and odor (EPA, 2005), contributors to which include particulate matter, hydrogen sulfide, methane, nitrogen oxide, volatile organics, and ammonia. Development of an EPA policy for monitoring and evaluating CAFO ammonia emission is currently underway, and it is probable that this effort will result in new requirements (EPA, 2005).
Incorporating ammonia recovery into an AD process would be beneficial for two key reasons. First, the cost of commercial fertilizers is continually increasing, and technologies that recover nitrogen for use as fertilizer would be advantageous. Second, the incorporation would enhance odor abatement and decrease ammonia release to the air. Unfortunately, currently available ammonia removal and recovery technologies cannot be widely applied to animal waste due to the high level of solids in manure wastewater, and the extremely high cost of the technology.
Some biological processes such as nitrification/denitrification are commonly used in wastewater treatment. However, these processes do not produce usable fertilizer. Researchers (Tilche et al., 2001, Choi et al., 2005) have studied full-scale nitrogen removal from piggery manure wastewater with Sequencing Batch Reactor (SBR) nitrification and denitrification without AD. Vanotti (2004) studied full-scale swine wastewater ammonia removal with another nitrification and denitrification system, Anoxic/Oxic (A/O). These processes are technically effective, as Szögi (2006) reported that the annual ammonia emissions were reduced 90% in the swine wastewater lagoon treated by using nitrification and denitrification. However, these aerobic processes require a large reactor and large amount of electricity for the ammonia oxidation step as well as for oxidizing the organic material to CO2. When treating AD effluent, a recently developed process, “anammox” (Fux and Siegrist, 2004) needed only 40˜50% oxygen compared with traditional nitrification and denitrification, and organic material was not required. However, there have been few successful demonstrations of either municipal or industrial wastewater treatment using this technology.
Several physio-chemical processes, including ion exchange and ammonia stripping, allow removal and recovery of ammonia. Ion exchange can be precluded as a viable option for animal waste treatment because it requires extremely low solids concentrations. However, ammonia scrubbing shows potential due to its ability to meet the animal manure needs regarding solids concentration and cost. Three common methods are 1) using a biofilter to oxidize ammonia to nitrate; 2) burning the gas to oxidize ammonia to nitrogen gas; and 3) water absorption for strong ammoniac industry wastewater. Unfortunately, all stripping processes developed to date are not optimized for animal manure. While other technologies exist for ammonia removal and nitrogen recovery, they are generally too expensive to be adopted by farmers.
In addition to nitrogen or ammonia recovery there is a need for associated technologies to remove or recover phosphorous (P) from the AD effluent as P has been identified as a major contributor to waterway water quality degradation through eutrophication (Stickney, 1994). To mitigate the effects of discharged P, environmental regulations have been established in many regions (Rosenthal, 1994; Bergheim and Brinker, 2003; MacMillan et al., 2003). For example, in Idaho, which is now becoming a large CAFO producer, P discharge is being actively debated as a potential regulation parameter for the incoming CAFOs and regulation is expected to occur very soon. Generally, P removal technologies for wastewater include chemical and biological processes. Biological methods are not suitable for animal manure due to the extremely high P content within the manures. Chemical methods include settling, flocculation, precipitation and electro-coagulation with one particular technology, struvite crystallization, receiving considerable recent attention. The formation of struvite (magnesium ammonium phosphate or MgNH4PO4.6H2O) requires the presence of three ions in solution, Mg2+, NH4+ and PO43− which react to form precipitates with low solubility (pKsp of 12.6) (Wrigley et al., 1992; 2002; Jeong and Hwang, 2005). One of the advantages of this process is that the struvite product can be utilized and marketed as a slow-release fertilizer and value-added product. A struvite crystallizer designed as a cone-shaped fluidized bed reactor was used to achieve high P removal (80%) from swine wastewater (Bowers and Westerman, 2005) and high P removal as struvite precipitation was also expected from digested dairy effluent. However, surprisingly poor P removal (<15%) was obtained under various conditions using this crystallizer (Zhang et al., 2006) for treating the effluent from an anaerobic digester, although better performance (50%) was observed treating dairy lagoon effluent. The results indicated that although organic P was converted to an inorganic form after anaerobic digestion, it was not available in an ionic form as is commonly believed. Instead, the majority of the P was in a suspended solid form, contained in particles smaller than 74 μm, with half or more in particles 2.5 μm or smaller. The high calcium content (about 1,000 mg/L) in the effluent may have contributed to the presence of P as a particulate by forming a calcium-phosphate suspended solid in the dairy effluent. Struvite crystallization requires dissolved reactive phosphate to form and the calcium-phosphate solids, with low solubility, provided little reactive phosphate, thereby blocking struvite crystallization, thus resulting in poor P reduction.
Unfortunately, physical solid-liquid separation processes for the removal of P associated with solids (e.g. sedimentation, screening, and filtration) generally have low efficiency because the majority of the solids are in fine particulate form in the manure wastewater (Zhang et al., 2006). Brownian motion and fine particle mass produce very slow sedimentation of the colloid particles in the water. Use of coagulants and flocculants can enhance the solids and P removal by aggregating fine particles to facilitate rapid settling and screening. Coagulants and/or flocculants destabilize and combine the suspended charged particles, resulting in larger particles or floc formation that separate more easily from the water. However, polymer usage would significantly increase process costs. For instance, the cost of the necessary chemicals is about $2.63 per 1,000 liter wastewater (Oh et al., 2005).
The prior art has thus-far failed to provide a system for the treatment of animal waste that recovers both nitrogen and phosphorous in a useful, commercially viable form, and that is economical enough to be employed by the farming community.